The invention relates to an active antenna with an array of radiating elements with a redundant architecture.
Various types of antennas are used to transmit or receive electromagnetic radiation, especially in the microwave frequency region, depending on requirements. The active antenna with an array of radiating elements is of special interest since it provides a transmit or receive pattern which can be configured at will; it can also generate several transmit or receive patterns.
The elements of the array of the same antenna are, generally, of the same type, that is to say they are all planar, horn-shaped, dipoles, helices, etc. The amplitude and the phase of the feed signal to each element determine the characteristics of the radiation pattern.
A beam-forming network is provided in order to feed the elements. This network includes phase shifters and possibly attenuators. The number of outputs of this array is equal to the number of elements. Each output is connected to an element by a power amplifier and, possibly, a filter. In the case of a receive antenna, a low-noise amplifier is provided instead of a power amplifier. The channel associated with a radiating element is termed an active subsystem.
The beam-forming network supplies each element with a signal whose amplitude and phase are matched to the desired radiation pattern.
In certain antennas, the programming of the beam-forming network can be modified. In this case, the network is said to be active. In the converse case (i.e. if the programming cannot be modified), the network is said to be passive.
Such antennas are used in particular on board vehicles, especially satellites or spacecraft. For on-board applications, such as space applications, it is necessary to limit the effects of any equipment failures on the performance of the antenna, i.e. to enable the antenna to meet specifications over a determined lifetime despite an anticipated occurrence of equipment failures.
The deterioration which may arise on account of equipment failures include, for a transmit antenna, a reduction in the Equivalent Isotropic Radiated Power (EIRP), for reception, the G/T ratio (where G is the gain and T the noise temperature of the antenna) and, in both cases, an upturn in the sidelobes, that is to say a deterioration in the specified transmit or receive pattern.
In general, equipment failures occur in the active subsystems.
Various solutions have been used hitherto to maintain the EIRP or the G/T coefficient of the antenna and its pattern within acceptable limits.
For transmit antennas, a first solution consists in uprating the power amplifiers and having them deliver a power greater than the specifications. However, excessive energy consumption is never desirable, especially in a space application. Moreover, with this solution it is not possible to correct the upturns in the sidelobes caused by equipment failures.
A second solution, which is applicable to transmission and to reception, consists in providing a number of radiating elements which is greater than what is strictly required. For example if, to meet the specifications, a number N of radiating sources is necessary, then a number Q is added to make provision for equipment failures. This solution also has the drawback of excess energy consumption at the beginning of the life of the antenna.
Moreover, if the beam-forming network is passive, that is to say if the amplitude and the phase of the feed signal to each element are not controllable, the initial performance must be better than what is strictly required so that the antenna can withstand equipment failures. This constraint necessitates uprating and usually cannot compensate for the effects of equipment failures; in particular, sensitive performance factors such as the level of the sidelobes are degraded with no possibility of compensation. If the beam-forming network is active, the antenna can be reconfigured after equipment failures; however, in this case, the abovementioned drawback of excess energy consumption at the beginning of the life of the antenna still remains.
In a third solution, the amplifiers and, optionally, the beam-forming network employ redundancy. For example, a number of reserve amplifiers are provided to replace defective amplifiers. In order for the wiring to remain within reasonable limits, in terms of simplicity and of bulk, each reserve amplifier can replace only a very limited number of active amplifiers. This constraint makes it impossible to minimize the number of reserve amplifiers. Moreover, replacement requires additional equipment such as switching elements, thus rendering the embodiment more complex and increasing the mass and the cost as well as the bulk. Although this solution theoretically makes it possible to maintain performance, it is not always satisfactory since the increase in mass and bulk are not optimal, especially for space applications.